1. Field of the Invention
This invention relates to the production of materials in the form of coatings or powders using a halogen-containing reactant which will react with a second reactant to form one or more reactive intermediates from which the powder or coating may be formed by disproportionation, decomposition, or reaction. In one embodiment, the powder or coating is formed directly from the one or more reactive intermediates. In a second embodiment, the one or more reactive intermediates react with a gaseous third reactant resulting in the formation of a powder or coating.
2. Description of the Related Art
Metallic and ceramic powders and coatings can be formed by a variety of techniques including vapor deposition, chemical vapor deposition (CVD), sputtering, dip coating, slurry painting, pack cementation, thermal spraying, grinding, milling, etc. Each technique has some advantages and disadvantages that dictate their major area of applications.
For example, when a powder of a particular material is to be formed, the material may be milled or ground. However, when the desired powder is an element such as aluminum, silicon, titanium, etc., such milling or grinding may require implementation of additional safety requirements. Additionally, the formation of powders from compounds usually requires additional steps to produce the compound from which the powder will be formed.
When a coating of a particular material is desired, the material may be applied as a vapor by first volatilizing the material, which may in some cases require very high temperatures, e.g. as high as 2000.degree. C. or higher for materials such as silicon and titanium, or the coating may be applied by other means mentioned above such as sputtering, coating, painting, or spraying which, while not requiring as high a temperature as the vaporization techniques, may not form a bond which is as satisfactory.
Stinton et al in "Advanced Ceramics by Chemical Vapor Deposition Techniques", Ceramic Bulletin, Volume 67, No. 2 (1988), pages 350-355, describes the use of CVD techniques to form a number of carbides, nitrides, oxides, and borides of various metals.
When a coating of a compound is desired, CVD is particularly well suited for many applications including deposition of semiconductor and metallic interconnects, deposition of hard coatings for tools and gears, and deposition of corrosion resistant coatings for aqueous and high temperature environments, Typically a gas mix such as, for example, TiCl.sub.4 and NH.sub.3 are passed over a substrate heated to high temperature, (&gt;1000.degree. C.) so that a film of TiN, which is formed in situ, deposits on the substrate.
Some of the factors which limit the use of CVD are cost (capital equipment cost and operating cost), and, in some cases, the need to operate at high temperatures that may degrade the properties of the substrate. Another limitation of CVD, in some cases, may be the difficulty in maintaining constant temperature on the substrate and constant composition of the gas phase, especially if the substrate has a convoluted geometry.
To reduce the effects of these disadvantages and limitations, improvement have been proposed in the past such as, for example, the use of plasma assisted CVD. Unfortunately, the use of such plasma assisted CVD requires working at low pressures which results in higher coating costs.
Other improvements aimed at improving the homogeneity of the coatings consisted of modifying the flowdynamics of the gas around the piece to be coated and careful design of the heating unit. Such approaches can result in expensive, one-of-a-kind systems, and low utilization of the reactants.
Another approach is described in Japka et al U.S. Pat. No. 4,623,400 wherein a gas mix containing vapors of the coating precursor is injected into a fluidized bed containing inert particles. The piece to be coated is immersed in the bed. The use of a flowing bed reactor (FBR) with its fast heat transfer permits the piece to be coated to be maintained at a uniform temperature by the bed, resulting in a more uniform coating. Unfortunately, in this process, much of the coating material is deposited on the inert fluidized bed particles and the reaction requires expensive reactants.
Using silicon as an example, when it is desired to form a powder compound such as silicon nitride or silicon carbide, it is not usually possible to form such compounds directly from silicon without the use of high temperatures. For example, Jennings, in an article entitled "On Reactions Between Silicon and Nitrogen", published in the Journal of Materials Science, Volume 18, (1963), pages 951-967, states that typical nitriding involves heating silicon, usually a powder compact, and nitrogen to temperatures between 1300.degree. and 1500.degree. C., although Pugar et al, in an article entitled "Low Temperature Direct Reactions Between Elemental Silicon and Liquid Ammonia or Amines for Ceramics and Chemical Intermediates", Materials Research Society Sympos ium Proceedings, Volume 21, 1988, pages 439-447, reports direct reactions between elemental silicon and either ammonia or hydrazine at low temperatures.
In the prior art, however, it has been conventional to use more reactive materials such as silicon halides or silanes as the source of silicon for forming silicon compounds such as silicon nitride, silicon carbide, etc. For example, Mazdiyasni et al U.S. Pat. No. 3,959,446 describes a process for producing silicon nitride powder using liquid silicon tetrachloride and ammonia as the initial reactants to form a mixture of silicon diimide and ammonium chloride which is then heated under a vacuum of from 10.sup.-3 to 10.sup.-4 torr to a temperature of 1200.degree. to 1350.degree. C. to form the silicon nitride from this intermediate.
Similarly, when a coating is to be formed comprising a compound, e.g., of silicon nitride or titanium carbide, it is necessary to both form the compound as well as provide a means for depositing or forming the coating on a substrate.
It would, therefore, be desirable to provide a process wherein powders or coatings comprising either a single element, or the reaction product of an element with another reactant, could be formed without the need for use of expensive reactants and/or high temperatures and/or pressures and the need for carefully controlled reaction conditions.